If your mother is intimidated by the idea of an
ECV, may I suggest that you rent one for a day at home. Select an area where the walking surface is good, as the pavement and curb cuts in WDW are very well maintained and scooter friendly. Take a walk with her (driving) outdoors, and, if possible, inside a good-sized mall. She may just surprise herself and love it! My 89 yr old mother fell in love within an hour, of being on WDW property. Of course, she was also a very experienced driver who really has missed the independence of driving herself. A big and most justified fear emerged quickly... tight crowds, particularly after dark. We developed a couple simple solutions - she could walk, as she still had plenty of energy and one of us would drive the scooter OR we would slip it in neutral and DH & I pushed it from either side while she sat back and didn’t have to worry.
A wheelchair is really not a bad thing to have along, as it can be pushed empty (except for all the stuff you may want to heap on it!!!) and your mother could take a ride as required. A lightweight, easily collapsed model doesn’t require much effort while using Disney transportation.
A third option for you might be a combination walker/transport chair. These are really neat. One would provide stability and support for your mom if she pushed it, a ready seat whenever she tired, and can convert to a transport chair so she could be pushed as needed. The downside is that they only have the small wheels, making them harder to push, and more sensitive to pavement irregularities but they are still quite manageable. We toured CAtalina Island,
Disneyland, and other area attractions quite easily, pushing my DM, in a transport chair when she was 94. She missed the ECV, though - LOL - but knew it was a bit beyond her at that point.
You are doing the smartest thing before your trip - research! Deciding on the most practical device for your needs will provide the maximum physical and mental comfort for your whole group. Good luck and have a wonderful trip!